Technological Field
The present invention relates to a fixing belt, a fixing device, and an image forming apparatus.
Description of Related Art
A fixing device employed for image forming apparatuses including copiers and laser beam printers generally brings a heated fixing belt into contact with a recording medium bearing an unfixed toner image thereon to fix the toner image onto the recording medium. In the fixing device, for example, one of two or more rollers supporting a fixing belt in an endless state is a heat roller that heats the fixing belt. The fixing device has excellent fixability by virtue of the relatively small heat capacity of the fixing belt, and the excellent fixability is advantageous, for example, for achievement of higher speed in formation of an image.
A technique for the fixing device is known, in which the fixing device includes a fixing belt with a trilayer structure including a base material, an elastic layer, and a surface layer, a larger curvature is imparted to the exit side of a nip portion formed by an upper pressure roller and the fixing belt (downstream side in a conveyance direction for a recording medium), and the surface properties of the fixing belt are improved by using fluororesin such as PFA for the material of the surface layer (e.g., Japanese Patent Application Laid-Open No. 2012-108545).
Further, the image forming apparatus is generally required to operate in high speed, and for the purpose it is effective to set the curvature of the fixing belt small at the nip portion, for example, set the diameter of a roller hanging the fixing belt thereon large, the roller provided at a position opposite to the pressure roller. However, a larger roller diameter leads to a smaller curvature in the exit side, and thus it is generally difficult to achieve higher speed (upsizing) in formation of an image and increase of the curvature in combination, and they are in trade-off relation.
It is preferred that the material of the surface layer be fluororesin, from the viewpoint of releasability. Modification of the surface properties of the surface layer, for example, enhancement of the releasability, generally depends on how many fluorine atoms (F groups) can be disposed on the surface of the surface layer, and this result determines the non-tackiness (high contact angle, low surface energy) of the surface layer. However, the surface layer made of fluororesin suffers from high tendency of the fluorine atoms to localize on the surface, with little room left for introduction of an F group or another substituent as an alternative to an F group on the surface. Accordingly, it is difficult to enhance the non-tackiness through modification of PFA. Thus, further study is needed to achieve separability and fixability in combination in a fixing belt including a surface layer made of fluororesin to meet the current requirement for higher speed in formation of an image.